A marine mud motor is a specialized marine motor used to propel a boat in shallow water applications. These motors are useful in marshes or other shallow water areas where the propeller frequently comes in contact with rocks, mud, logs, and weeds, etc., at the bottom of the body of water. The design of the motor and drive shaft propels boats in extreme conditions by allowing the propeller to ride gently over these obstacles with minimal damage to the drive.
A marine mud motor, illustrated in FIG. 1, includes a frame 8 that supports an engine 14. The frame is mounted on a boat 10 by an engine mount 30. A long drive tube 16 is coupled to the frame by supports 28 and the drive tube contains a drive shaft 26. A propeller 22 is connected to the drive shaft and moves a boat forward. This design has been used for more than 30 years and is in use throughout the U.S. and Asia today to propel boats through adverse shallow water conditions.
The conventional design described above has a long drive tube 16 that encases a drive shaft 26 which is three to seven feet in length. This design includes a drive shaft, which is supported during rotation by bronze or composite bushings pressed into the drive tube, one on the bottom end and one on the top end of the drive tube. The bushings are contained in the drive tube assembly 16. Marine mud motors using bronze or composite bushings pressed into the drive tube have been in production around the world for over 30 years.
As with any device operated under water, moisture is a serious problem. Seals are used to keep water out and lubricants inside the drive. Because bronze and composite bushings do not corrode easily, they have conventionally been used as a wear surface in the drive. Roller bearings have been tried, but have repeatedly failed due to corrosion and alignment difficulties associated with this conventional design. The use of roller bearings has failed due to the extreme conditions in which the motors are used, and the eventual intrusion of water, dirt and sand, which quickly rusts and seizes the bearing, leading to extremely early failure.
To avoid this early drive shaft failure, multiple seals conventionally are used and the drive tube is filled completely with grease. Even with multiple seals and filling the tube with grease, water, dirt, and particles enter the drive and cause early bearing failure. This early bearing failure usually takes less than two years, even with constant attention and lubrication. For example, users must frequently fill the drive tube with grease, which tends to leak out.
Another significant problem is due to the drives' inherent design. The rotating shaft causes a Venturi effect which draws water up inside the drive tube past the multiple seals. Because the drive is in frequent contact with the bottom of the body of water, silt and sand accompany the water, which is pulled into the drive tube. Of course, the silt, particles and sand significantly accelerate bushing and drive tube wear and induce early failure.
Even more water and silt is pulled into the drive tube when the drive is at rest in the water. This is caused by cooling parts that create a vacuum, which draws water inside the drive. This additional water, silt and sand further contribute to the problem of the Venturi effect.
Because of these problems, bronze bushings, composites and even ceramics have been used as a wear surface for the rotating drive shaft. Other bearings have been tried, but the sand, dirt and water wear them out in a very short time. Accordingly, the use of bronze or composite bushings in the drive tube has been the industry standard.
Conventional drive tubes are completely, or nearly completely, filled with grease which lubricates both the top and bottom drive bushings. This lubrication helps combat any moisture that enters the system and helps to hydraulically balance the long drive shaft, which tends to vibrate during operation due to its length. Seals mounted on each end of the drive tube keep the grease inside the tube. The lower end of the drive tube supports the propeller and commonly features multiple seals, usually three. One of the seals is used to keep the grease inside the drive tube and two of the three seals are reversed, thus keeping water from entering the drive tube. Multiple seals are used because the long drive tube causes a Venturi effect, which attempts to draw water inside the drive tube. The two reversed seals slow this "pumping" process down. Regardless, the Venturi effect is strong and eventually small amounts of water with silt enter the system and cause accelerated wear and contamination of the lubricant. For this reason, bearings have not been used successfully because the moisture and dirt causes early bearing failure. Thus, only bushings have been used for the past 30-plus years.